<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      width: 300px;
      height: 300px;
    }
  </style>
</head>

<body>
  <div></div>
  <script>
    function getRandomColor(flag) {
      let arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f']
      let str = '#'
      if (flag) {
        for (let i = 0; i < 6; i++) {
          str += arr[Math.floor(Math.random() * arr.length)]
        }
        return str
      } else {
        let r = Math.floor(Math.random() * 256)
        let g = Math.floor(Math.random() * 256)
        let b = Math.floor(Math.random() * 256)
        return `rgb(${r},${g},${b})`
      }
    }

    console.log(getRandomColor(true))   //#ffffff
    console.log(getRandomColor(false))   //rgb(255,255,255)

    const divv = document.querySelector('div')
    divv.style.backgroundColor = getRandomColor(true)
  </script>

</body>

</html>